Electron gun for color cathode ray tubes with side electron-beam-passing apertures of plurality of circular arcs having different radii of curvature

ABSTRACT

An in-line electron gun for a colored cathode ray tube has an electronic field correction plates with a central key-hole shaped aperture with two opposite straight lines and two side apertures each with a plurality of vertical and horizontal in addition to a plurality of circular arcs of different radii.

TECHNICAL FIELD

The present invention relates to an in-line electron gun for a colorcathode ray tube and, more particularly, to an electric field correctionplate for correcting aberration such as astigmatism in a main lens ofthe electron gun, in which a central aperture is of a key-hole shape andside apertures are of a asymmetric circular shape so as to make up foran assembly error caused by the application of the prior art ellipticalshape to the main lens, thereby improving the assembly characteristicsand the focusing characteristics.

BACKGROUND ART

Recently, in order to improve resolution characteristics of an in-lineelectron gun for a color cathode ray tube, the prior art separate typemain lens becomes changed to the common type rim lens, and an enlargedaperture becomes applied to the main lens.

In this electron gun having the enlarged aperture, the difference in anopening size between the horizontal and the vertical direction causesthe main lens to become asymmetric and the focusing forces to becomedifferent between the horizontal and the vertical directions, therebycausing aberration and distorting electron beams. That is, there is aproblem that astigmatism and coma aberration occur in the main lens andthus exert a bad influence on the focusing characteristics.

In recent, an arrangement for compensating for such aberration is widelyused wherein electric field correction plates are applied to the largeaperture type main lens.

In a common electric field correction plate as shown in FIG. 3, threeapertures 11 are arranged in line and the shapes of the apertures 11 areelliptical, which control the intensity of lens between the horizontaldirection and the vertical direction by the change of the ellipticalshape, and thus compensate for the aberration in the main lens. Forclearly correcting the aberration of the main lens and improving theperformance of the electron gun, the electric field correction plates 9,10 must be assembled in the electron gun with high precision.

However, it is very difficult to assemble such electric field correctionplate with high precision since the apertures in the electric fieldcorrection plates are elliptical, thereby deteriorating the wholefocusing characteristics of the electron gun according to a lowering ofthe assembly precision.

To alleviate such a problem in Japanese Patent Hei 6-75378, only thecentral aperture of the electric field correction plate is ellipticaland opposite side apertures of the electric field correction plate arecircular, thereby improving the assembly characteristics. However, it isvery difficult to correct astigmatism of opposite side beams by suchelectric field correction plate. Otherwise, there is a problem that themain lens should become complicated in the structure as compared withthe prior art main lens.

Meanwhile, the in-line electron gun for a color cathode ray tubedisclosed in U.S. Pat. No. 4,583,024, issued on Apr. 1, 1986, comprisesa main focus lens formed by two spaced electrode members, each havingthree separate inline apertures therein, a central aperture and two sideapertures. The improvement comprises each of the apertures in each ofthe focus lens electrodes having a shape that distorts a portion of thefocus lens thereat, to at least partially compensate for an astigmaticeffect without any further additional part. The side apertures in bothof the electrodes are nonsymmetrical about axes that pass through therespective side apertures and are perpendicular to the initial coplanarpaths of the electron beams. In this case, astigmatism is partiallycompensated for, but little effect can be obtained.

U.S. Pat. No. 4,833,364, issued to Izumida, et al., on May 23, 1989discloses an electron gun for color picture tubes, wherein a focus gridand an anode made of elongated plates serves as main lens electrodes,and three apertures of the focus grid and the anode constitute a mainlens assembly. The three apertures of each of the elongated plates havea central aperture defined by two first curves arcuated outward, and twoside apertures each of which is defined by a second curve as an innerhalf arcuated inward and a third curve as an outer half arcuatedoutward, the first and second curves being less arcuated than the thirdcurve. Elliptical rings are bonded to the elongated plates such that theend portions of the rings are constituted by semicircular portionsaligned with those of the side apertures, thereby preventingastigmatism.

However, in such constitution, the shape of the inner wall in the ringelectrodes and the shape of the elongated plates should be changed tocontrol the path of the electron beams, thereby causing the change ofthe tube size to be difficult. That is, the anode voltage and theintensity of the main lens should be changed for the larger size tubes,but it is very difficult to change the path of the electron beams onlyby the change of the shape of the elongated plates since the cylindricalinner wall in the ring electrodes serves as an static electric lenselectrode.

DISCLOSURE OF INVENTION

The present invention has been made to overcome the above describedproblems of the prior arts, and accordingly it is an object of thepresent invention to provide an in-line electron gun for a color cathoderay tube, which improves the assembly characteristics and compensatesfor aberration such as astigmatism, thereby substantially improving thefocusing characteristics over the whole phosphor screen.

To achieve the above object, the present invention provides an in-lineelectron gun for a color cathode ray tube, which includes an electronbeam generating means for generating and directing three electron beamsalong paths toward a phosphor screen, and a main lens of a largeaperture type for focusing the three electron beams radiated from saidelectron beam generating means, said in-line electron gun for a colorcathode ray tube being characterized in that said main lens is formedbetween a focusing electrode and an accelerating electrode; and electricfield correction plates are disposed in each of the focusing electrodeand an accelerating electrode in the vertical direction to electron beampassing axes, and have three electron beam passing apertures aligned inline, respectively, said three apertures comprising at least one centralelectron beam passing aperture of a key-hole shape which is symmetricalwith respect to a vertical line to a central electron beam passing axisand two side electron beam passing apertures each of which is defined bya plurality of circular arcs having different radii of curvature,connected to each other circular arc, and asymmetrical with respect to avertical line to each side electron beam passing axis.

It is preferred that each of said two side electron beam passingapertures of the electric field correction plates has the circular arctoward the central electron beam passing aperture smaller in radius ofcurvature than the circular arc far from the central electron beampassing aperture, and said plurality of circular arcs of the sideelectron beam passing apertures are connected by even-numbered straightlines.

Also, it is preferred that each of said two side electron beam passingapertures of the electric field correction plates has a shape of akey-hole toward the central electron beam passing aperture and a shapeof an arcuate curve far from the central electron beam passing aperture.That is, each of said two side electron beam passing apertures of theelectric field correction plates has a shape of a key-hole at one sidewith respect to a vertical line to each side electron beam passing axisand a shape of an arcuate curve at other side with respect to a verticalline to each side electron beam passing axis.

Furthermore, each radius of the circular arcs constituting said threeelectron beam passing apertures of the electric field correction platedisposed in the focusing electrode is preferrably smaller than eachradius of the circular arcs constituting said three electron beampassing apertures of the electric field correction plate disposed in theaccelerating electrode, and each of said three electron beam passingapertures of the electric field correction plate disposed in thefocusing electrode may have a shape of a key-hole.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing the structure of an in-line electrongun for a color cathode ray tube embodying the present invention.

FIG. 2A is a sectional view showing the structure of main lenselectrodes embodying the present invention.

FIG. 2B is a front elevational view of one of the lens electrodes.

FIG. 2C is a side view partly in section of the electrode of FIG. 2B.

FIG. 3 is a plan view of the prior art electric field correction plateconstituting a main lens.

FIGS. 4A and 4B are plan views of electric field correction platesconstituting a main lens according to one embodiment of the presentinvention.

FIG. 5 is a plan view of one electric field correction plateconstituting a main lens according to another embodiments of the presentinvention.

FIG. 6A is shape diagrams of the conventional electron beam spots andFIG. 6B shape diagrams of electron beam spots according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail with reference to theaccompanying drawings.

Each shape of the central electron beam passing apertures 12 and 14 andtwo side electron beam passing apertures 13 and 15 of electric fieldcorrection plates 9 and 10 according to one embodiment of the presentinvention is shown in FIGS. 4A and 4B, and each shape of a centralelectron beam passing aperture 16 and two side electron beam passingapertures 17 of the electric field correction plate 9 according toanother embodiment of the present invention is shown in FIG. 5.

In FIG. 4A the electric field correction plate 9 has three electron beampassing apertures 12, 13 and 13 aligned in line, and in FIG. 4B theelectric field correction plate 10 has three electron beam passingapertures 14, 15 and 15. Each of said three electron beam passingapertures 12, 13 and 13, and 14, 15 and 15 comprises at least onecentral electron beam passing aperture 12 or 14 of a key-hole shapewhich is symmetrical with respect to a vertical line to a centralelectron beam passing axis, and two side electron beam passing apertures13 and 13, or 15 and 15 defined by a plurality of asymmetric circulararcs, thereby improving the assembly precision. Also, the focusingcharacteristics over the whole phosphor screen are substantiallyimproved since aberration such as astigmatism, coma aberration, etc.,caused in the electron gun is substantially compensated for due to saidplurality of asymmetric circular arcs of the side electron beam passingapertures 13 and 13, or 15 and 15.

FIG. 1 shows the structure of an in-line electron gun for a colorcathode ray tube embodying the present invention. A main lens comprisinga focusing electrode 6 or a fifth electrode G5 and an acceleratingelectrode 7 or a sixth electrode G6 in FIG. 1 is shown in FIGS. 2A to 2Cin detail and each with an open passage 8.

In FIG. 1, the in-line electron gun comprises three spaced inlinecathodes 1 for emitting electron beams, a first electrode G1 or acontrol electrode 2, a second electrode G2 or a screen electrode 3, athird electrode G3 or a focusing electrode 4, a forth electrode G4 or ascreen electrode 5, a fifth electrode G5 or a focusing electrode 6, anda sixth electrode G6 or an accelerating electrode 7.

With this arrangement, an electric voltage between 0V and around 100V isapplied to the cathodes 1, the first electrode 2 is grounded, anelectric voltage EC3 between 400V and 800V is applied to the second andthe forth electrodes 3 and 5, an electric voltage EC2 between 400V and800V is applied to the second and the third electrodes 3 and 5, anelectric voltage Eb between 5 KV and 8 KV is applied to the third andfifth electrodes 4 and 6, and an electric voltage between 20 KV and 30KV is applied to the sixth electrode 7.

Although the electron beams are focused at the central portion of thephosphore screen in the conventional electron gun, the electron beamsare not converged to one point at the peripheral portion of thephosphore screen due to the difference of the curvatures of a panel andthe distances of the electron beam paths from the central portion, andthe spots of the electron beams are enlarged due to the difference ofthe lens magnifications in the rim main lens of the focusing electrode 6and the accelerating electrode 7, thereby degrading the resolution of animage.

To compensate for such asymmetry of the electric field, electric fieldcorrection plates are adopted in the prior art electron gun and theapertures 11 thereof become elliptical as shown in FIG. 3, therebyforming a new asymmetrical lens and correcting aberration of the rimmain lens in reverse. However, an eccentric assembly jig is required forassembling such electron gun. Still, it is difficult to assemble suchelectric field correction plates with desirable high precision sincerods of the assembly jig are circular, thereby failing to improve theresolution of the electron gun according to the assembled position ofthe electric field correction plates.

Accordingly, in the present invention made to overcome the abovedescribed problems of the prior arts, one central electron beam passingaperture 12 or 14 of the electric field correction plates 9 and 10 has akey-hole shape as shown in FIGS. 4A and 4B, and two side electron beampassing apertures 13 or 15 of the electric field correction plates 9 and10 have of shape defined by a plurality of asymmetric circular arcs.

That is, said main lens is formed between a focusing electrode 6 and anaccelerating electrode 7 as shown in FIGS. 1 and 2. The electric fieldcorrection plates 9 and 10 are disposed in each of the focusingelectrode 6 and an accelerating electrode 7 in the vertical direction toelectron beam passing axes. In FIG. 4 and 5, the electric fieldcorrection plates 9 and 10 have three electron beam passing aperturesaligned in line, respectively. Said three apertures comprise at leastone central electron beam passing aperture 12 or 14 of a key-hole shapewhich is symmetrical with respect to a vertical line to a centralelectron beam passing axis, and two side electron beam passing apertures13 or 15, each of which is defined by a plurality of circular arcshaving different radii of curvature, connected to each other circulararc and asymmetrical with respect to a vertical line to each sideelectron beam passing axis.

FIG. 5 shows apertures 16 and 17 in the electric field correction plate9 constituting a main lens according to another embodiments of thepresent invention, which are different in shape form, but act as thesame function as, the apertures 12 and 13 in the electric fieldcorrection plate 9 as shown in FIG. 4A.

In detail, the side electron beam passing apertures 15 of the electricfield correction plate 10 as a divergence lens electrode disposed in thesixth electrode 7 is made larger in the radius of the curvature of thecircular arcs than the side electron beam passing apertures 13 and 17 ofthe electric field correction plate 9 as a focusing lens electrodedisposed in the sixth electrode 6, thereby becoming a system forcontrolling, and compensating for, the aberration and facilitating thecompensation for the aberration of the side electron beams as suchasymmetric lens. Thus, the focusing performance becomes improved overthe whole phosphore screen. Furthermore, each of said two side electronbeam passing apertures 13 and 15 of the electric field correction plates9 and 10 may have the circular arc toward the central electron beampassing aperture 12 and 14 smaller in radius of curvature than thecircular arc far from the central electron beam passing aperture 12 and14.

With the electric field correction plates 9 and 10 according to theabove embodiments of the present invention, the assembly precision bythe prior art assembly jig are greatly improved as compared with theassembly of the prior art electron gun because the electron beam passingapertures comprise circular arcs, thereby facilitating the precisecorrection of aberration by means of the circular electron beam passingaperture and the side electron beam passing apertures 13 or 15 of theasymmetrical radii, and further facilitating the design of the mainlens.

In FIGS. 4A4B and 5, said side electron beam passing apertures 13, 15and 17 are formed by connecting said plurality of circular arcs byeven-numbered straight lines. Furthermore, each of said side electronbeam passing apertures 13, 15 and 17 of the electric field correctionplates 9 and 10 has a shape of a key-hole close to the central electronbeam passing aperture 12, 14 or 16, and a shape of an arcuate curveremote from the central electron beam passing aperture 12, 14 or 16 asshown in FIGS. 4A, 4B and 5. That is, the shape of each of the sideelectron beam passing apertures 13, 15 and 17 is formed of a key-holeshape at one side with respect to a vertical line to each side electronbeam passing axis, and of an arcuate curve shape at other side withrespect to the vertical line. All three electron beam passing apertures12 and 13 of the electric field correction plate 9 disposed in thefocusing electrode 6 may be made of a keyhole shape.

In the meantime, each radius of the circular arcs constituting saidthree electron beam passing apertures of the electric field correctionplate 9 disposed in the focusing electrode 6 may be selected smallerthan each radius of the corresponding circular arcs constituting saidthree electron beam passing apertures of the electric field correctionplate 10 disposed in the accelerating electrode 7.

As described above, by forming the three electron beam passing aperturesof the electric field correction plates 9 and 10 with key-hole shapesand asymmetric circular arc shapes in the in-line electron gun for acolor cathode ray tube according to the present invention, the assemblycharacteristics are greatly improved and aberration of the main lens iscompensated for by means of the asymmetrical circular arcs as comparedwith the prior art three electron beam passing apertures 11 of anelliptic shape. Thus, the focusing characteristics over the wholephosphor screen become largely improved.

Moreover, the charge and voltage-resistance characteristics are alsoimproved. In the prior art main lens electrode, a large quantity ofburrs are formed by putting the piercing direction of the aperture inreverse for enlarging the Vertical aperture due to the decrease of thespheric aberration, thereby deteriorating voltage-resistancecharacteristics. In the present invention, it can be improved becausethe piercing direction of the aperture can be changed. Also, since noburr is formed in the main lens electrode and the electrode is close tothe inner wall of the neck, some electrons are drifted through a gap ofthe main lens to the inner wall of the neck, thereby causing the chargedrift. In the present invention, such charge drift becomes removed byadding a burring of about 1.2 mm length to the main lens electrode.

Also, in the focusing grade quality, astigmatism could be decreased byenlarging the size of the core of the electron beam in the horizontaland the vertical directions and changing the shape of the astigmaticplate electrode. And the size of the spot in the horizontal and thevertical directions can become optimized by changing a gap between theelectrodes.

The above-mentioned improvement is applied in actual practice with eachcathode current of I_(x)=120 μA and I_(x)=200 μA, which results arecompared with the prior art electron gun in the following table:

SPOT SIZE (mm) FOCUS CENTER BEAM OUTER BEAM VOLTAGE (Volt) 5% EDGE 5%EDGE Centroid I_(K) = 120 μA (the prior art) 5700 1.0178/0.450 1.716/0.908 10126/0.481  1.676/1.063 4.86 5800 0.949/0.361 1.425/0.8710.912/0.422 10445/1.081  4.87 5900 0.728/0.309 1.128/0.863 0.723/0.3941.211/1.055 4.89 6000 0.507/0.353 0.873/0.833 0.564/0.419 1.000/1.0114.90 6100 0.311/0.394 0.628/0.877 0.452/0.437 0.775/0.974 4.92 62000.21510.438 0.409/0.908 0.409/0.460 0.711/0.994 4.93 6250 0.233/0.4600.350/0.930 0.381/0.515 0.646/1.066 4.94 6300 0.275/0.483 0.336/1.0210.395/0.546  0.604/1.1.04 4.95 I_(K) = 120 μA (the present invention)6650 0.446/0.486 0.793/1.211 4.87 6700 0.236./0.446  0.610/1.1180.370/0.460 0.721/1.267 4.88 6750 0.228/0.426 0.550/1.111 0.314/0.4810.648/1.334 4.89 6800 0.205/0.405 0.478/1.121 0.327/0.481 0.572/1.3684.89 6850 0.226/0.398 0.390/1.154 0.321/0.497 0.505/1.391 4.90 69000.251/0.409 0.328/1.182 0.343/0.520 0.486/1.406 4.91 6950 0.295/0.4350.356/1.199 I_(K) = 200 μA (the prior art) 6100 0.322/0.582 0.918/0.8920.417/0.560 0.807/1.064 4.92 6150 0.242/0.600 0.749/0.927 0.413/0.5890.757/1.018 4.93 6200 0.263/0.637 0.617/0.906 0.407/0.617 0.707/1.0504.94 6250 0.291/0.672 0.484/.0932 0.452/0.641 0.677/1.022 4.95 I_(K) =200 μA (the present invention) 6650 0.276/0.401 0.653/1.072 0.351/0.5480.789/1.073 4.87 6700 0.232/0.426 0.535/1.092 0.330/0.579 0.687/1.1124.88 6750 0.259/0.449 0.444/1.100 0.356/0.597 0.578/1.127 4.89 68000.290/0.484 0.374/1.114 0.379/0.626 0.501/1.461 4.90 6850 0.344/0.5150.424/1.081

FIG. 6A is shape diagrams of the conventional electron beam spots, whichrepresent weak focusing. In FIG. 6B showing shape diagrams of electronbeam spots according to the present invention, the core portions of theelectron beams are increased and the halo portions of the electron beamsare decreased, thereby greatly improving the focusing and spotcharacteristics as compared with the prior art electron gun.

As described above according to the embodiments of the presentintention, the assembly characteristics of the electron gun and thefocusing characteristics over the whole phosphor screen through thereverse compensation for aberration of the main lens are greatlyimproved by means of the central electron beam passing aperture 12 or 14of a key-hole shape and two side electron beam passing apertures 13 or15 of a asymmetric circular shape in the electric field correctionplates 9 and 10.

While the present invention has been particularly shown and describedwith reference to the particular embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be effected therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. An in-line electron gun for a color cathode raytube, which includes an electron beam generating means for generatingand directing three electron beams along paths toward a phosphor screen,and a main lens of a large aperture type for focusing the three electronbeams radiated from said electron beam generating means, said in-lineelectron gun for a color cathode ray tube being characterized in that:said main lens is formed between a focusing electrode and anaccelerating electrode; and electric field correction plates disposed ineach of the focusing electrode and an accelerating electrode in thevertical direction to electron beam passing axes, and have threeelectron beam passing apertures aligned in line, respectively, saidthree apertures comprising at least one central electron beam passingaperture of a key-hole shape which is symmetrical with respect to avertical line to a central electron beam passing axis and which includestwo opposite straight lines, and two side electron beam passingapertures each of which is defined by a plurality of circular arcshaving different radii of curvature, connected to each other circulararc by a plurality of straight lines including at least two opposite andparallel straight lines and asymmetrical with respect to a vertical lineto each side electron beam passing axis.
 2. An in-line electron gunaccording to claim 1, wherein each of said two side electron beampassing apertures of the electric field correction plates has a shape ofa key-hole toward the central electron beam passing aperture which hasthe circular arc smaller in radius of curvature, and a shape of anarcuate curve larger in radius of curvature far from the centralelectron beam passing aperture.
 3. An in-line electron gun according toone of claim 1, wherein each radius of the circular arcs constitutingsaid three electron beam passing apertures of the electric fieldcorrection plate disposed in the focusing electrode is smaller than eachradius of the circular arcs constituting said three electron beampassing apertures of the electric field correction plate disposed in theaccelerating electrode.
 4. An in-line electron gun according to one ofclaim 1, wherein each of said three electron beam passing apertures ofthe electric field correction plate disposed in the focusing electrodehas a shape of a key-hole.